This invention relates to shrink films based on selected linear, low density copolymers of ethylene with certain .alpha.-olefins, which films have outstanding optical properties and a good balance of other physical properties and shrink properties.
Shrink films of oriented polyethylene and various copolymers of ethylene are well known; see, for example, U.S. Pat. Nos, 3,299,194 to Golike and 3,663,662 to Golike et al.
A polyolefin shrink film, used mainly for wrapping food products and a variety of consumer goods, should have good optical clarity; otherwise, the consumer appeal of the packaged article within the wrapping would be diminished or lost. For practical applications, the film should shrink within a temperature range of approximately 100.degree. to 120.degree. C. to a degree of at least 15% in the direction of orientation and with sufficient force to provide a tight-fitting skin around the article enclosed within the wrapping. The film also should have good mechanical properties, such as tensile strength and modulus, so that it will stretch and then shrink without tearing, will maintain good physical contact with the packaged article at all times, and will not get easily damaged in handling.
One prior art technique for making ethylene polymer shrink films required polymer crosslinking prior to stretching in order to impart to the film greater mechanical strength. This crosslinking usually was accomplished by irradiation with high energy particles or with gamma rays.
In order to obtain a resin composition yielding films with satisfactory properties for shrink film applications without crosslinking prior to stretching, it has been generally necessary in the past to blend low density and high density ethylene polymers. Naturally, it would be desirable to be able to make shrink films from a single low density ethylene polymer resin. In this context, the term "low density" means 0.940 g/cm.sup.3 or less, and "high density" means more than 0.940 g/cm.sup.3.
A recent commercial offering of the Dow Chemical Company, DOWLEX.RTM. low density "polyethylene" resins, are described in a Dow bulletin as giving blown film having excellent optics and superior strength properties. Yet, the same bulletin indicates that these resins are not suitable for making shrink films because they will shrink less than conventional low density polyethylene film and will shrink within a narrower temperature range. DOWLEX.RTM. resins are in fact copolymers of ethylene with 1-octene. Although the bulletin does not explain by what technique DOWLEX.RTM. experimental shrink films were made, it is believed that they were irradiated prior to stretching. Conventional low density polyethylene films must be crosslinked prior to stretching, and prior irradiation was the usual crosslinking technique practiced by the shrink film industry.
U.S. Pat. No. 4,205,021 to Morita et al. discloses a class of ethylene/.alpha.-olefin copolymers having long-chain branchings, as reflected by their low value of g.eta.=[.eta.]/[.eta.].sub.l of 0.05-0.78, wherein [.eta.] is the intrinsic viscosity of the copolymer and [.eta.].sub.l is the intrinsic viscosity of a linear polyethylene having the same weight average molecular weight. Those copolymers are said to give films having good transparency, while copolymers having high g.eta., 0.80 to 1.0, are said to have poor transparency. Morita et al. are not concerned with shrink films, however.